Olson's theorem for cyclic groups

Let $n$ be a large number. A subset $A$ of $Z_n$ is complete if $S_A = Z_n$, where $S_A$ is the collection of the subset sums of $A$. Olson proved that if $n$ is a prime and $|A|> 2n^{1/2}$, then $S_A$ is complete. We show that a similar result for the case when $n$ is a composite number, using a different approach.